The present invention concerns a method and device for inserting a supplementary item of information, such as a secret watermark, in digital data.
It also concerns a method and device for decoding such a supplementary information item inserted by the insertion method according to the invention.
It falls in general terms within the technical field of watermarking of digital data.
The proliferation of exchanges of digitized multimedia data by computer is assisting the creation and distribution of illicit copies and, in general terms, the illegal manipulation of data.
In order to limit this illegal manipulation, watermarking of the digital data is used, which consists of inserting a watermark directly in the digital data, for the purpose, for example, of identifying the owner, the sender or the addressee of the data. Inserting this watermark is similar to the coding of an additional information item in the digital data.
A conventional watermarking consists of inserting a visible logo when the digital data are images. Nevertheless, this watermark is easy to remove for a user who wishes to illegally manipulate this image.
Use is then frequently made of a watermark which must exhibit the following quality factors:
Said watermark must be imperceptible, that is to say the insertion of such a watermark must preserve the perceptual quality of the digital data, for example the visual quality for images or the auditory quality for audio data. The imperceptibility of the watermark also makes it more difficult to pirate it.
Said watermark must also be indelible, that is to say be statistically undetectable in the watermarked digital data so as to resist intentional attacks for destroying the watermark.
Said watermark must also be robust vis-a-vis conventional processing applied to digital data, such as compression and decompression, digital/analog conversion, filtering etc.
Finally, said watermark must be reliable, that is to say must allow a reliable decision with regard to the existence or otherwise of a given watermark in given digital data.
Known watermarking methods apply to non-compressed images. However, in many practical cases, a compressed image is at hand, for example delivered by a digital photographic apparatus.
It is therefore advantageous to insert a watermark in compressed data.
The document U.S. Pat. No. 5,727,092 proposes for example a method of inserting additional data in a set of compressed data. The method includes an entropic decoding of the set of compressed data, followed by the insertion of additional data in the set of decoded data, and then an entropic recoding of the set of watermarked data.
The additional data which were inserted were binary data, where the number of bits, and the insertion xe2x80x9clocationxe2x80x9d depend on the particular statistical properties of the set of compressed data which is being processed.
To this end, during a prior analysis step, a search is made in the set of compressed data which is being processed, for similar coefficient pairs, that is to say ones having values close together and similar frequencies of occurrence. These pairs, specific to the set of data being processed, are used for inserting the additional data.
A key representing the pairs used must be added to the coded data in order to be able to subsequently extract the additional information.
This method consequently entails operations on all the coefficients of the set of data to be watermarked. These operations require time and memory space.
In addition, the result is not robust to subsequent operations, such as filtering, which can introduce errors.
The document xe2x80x9cRobust image watermarking in the sub-band or discrete cosine transform domainxe2x80x9d, which appeared in the conference proceedings EUSIPCO 98 (Sep. 8-12, 1998), pages 2285-2288, proposes a watermarking method which is robust to compression. This method uses non-quantized low-frequency coefficients of the signal to be watermarked, which is not compressed.
This method is advantageously used where the low-frequency coefficients are subsequently quantized very finely compared with all the other sub-bands, or even are not quantized at all. Compression is effected after watermarking.
These two documents show that, up to the present time, it was not envisaged to effect a robust watermarking on compressed data, the constraints peculiar to each of these two aspects being different.
The present invention aims to remedy the drawbacks of the prior art, by providing a method and a device for inserting an additional information item in compressed digital data, which are both more robust and more rapid than those of the prior art.
To this end, the invention proposes a method of inserting an additional information item in a set of digital data representing physical quantities, the data having been processed by transformation, quantization and entropic coding in order to be compressed, characterised in that it comprises the steps of:
extracting a subset of low-frequency coefficients, from the set of data,
entropic decoding of the coefficients of said subset,
modulation of the decoded coefficients by a signal representing the additional information, so as to form quantized watermarked coefficients,
entropic coding of the watermarked coefficients,
insertion of the watermarked encoded coefficients in place of the coefficients of the subset, in the set of data.
Correlatively, the invention proposes a device for inserting an additional information item in a set of digital data representing physical quantities, the data having been processed by transformation, quantization and entropic coding in order to be compressed, characterised in that it comprises:
means of extracting a subset of low-frequency coefficients, from the set of data,
means of entropic decoding of the coefficients of said subset,
means of modulation of the decoded coefficients by a signal representing the additional information, so as to form quantized watermarked coefficients,
means of entropic coding of the watermarked coefficients,
means of insertion of the watermarked encoded coefficients in place of the coefficients of the subset, in the set of data.
The method and device according to the invention make it possible to insert an additional information item in compressed digital data, rapidly, whilst offering good degree of robustness.
The watermarking signal is inserted on a sub-sampled version of the image, which makes it possible to form only a partial entropic decoding of the set of compressed data.
The number of operations performed on the compressed data is thus limited. The calculation complexity remains low.
The inventors have shown that the processing carried out according to the invention is robust. This is notably due to the fact that it is performed on quantized coefficients, which undergo no dequantization. The method according to the invention is therefore totally independent of any quantization operation.
In addition, the invention presents the advantage of substantially preserving the size of the compressed file, which therefore makes it possible to add information without losing the advantage of compression.
According to a preferred characteristic, the signal representing the additional information is a pseudo-random signal. This type of signal makes it possible to insert secret information, which is difficult to detect or imitate.
According to another preferred characteristic, the modulation step includes a spectral breakdown of the set of low-frequency coefficients in order to extract therefrom a second subset of low-frequency coefficients, the modulation of the coefficients of the second subset by the signal representing additional information, and a transformation, which is the reverse of the spectral breakdown, of the subset of low-frequency coefficients containing the second subset of modulated coefficients.
Thus the insertion of the watermarking is effected over a limited number of coefficients. Preferably, the extracted subset is the subset of the coefficients with the lowest frequency resulting from the breakdown. Thus the insertion of the watermarking is effected on the very low frequencies of the image, which improves still further the robustness and imperceptibility of the watermarking.
The invention also concerns a method of detecting an additional information item in a set of digital data representing physical quantities, the data having been processed by transformation, quantization and entropic coding in order to be compressed prior to the insertion of the additional information, characterised in that it comprises the steps of:
extracting a subset of low-frequency coefficients, from the set of data,
entropic decoding of the coefficients of the said subset,
calculation of the correlation between the said subset and a signal representing the additional information, and
comparison of the result of the correlation with a threshold.
The invention also concerns a device for detecting an additional information item in a set of digital data representing physical quantities, the data having been processed by transformation, quantization and entropic coding in order to be compressed prior to the insertion of the additional information, characterised in that it comprises:
means of extracting a subset of low-frequency coefficients, from the set of data,
means of entropic decoding of the coefficients of the said subset,
means of calculation of the correlation between the said subset and a signal representing the additional information, and
means of comparison of the result of the correlation with a threshold.
The detection method and device make it possible to detect whether or not a given watermarking is present in the processed data.
The invention also relates to a digital signal processing apparatus, such as a digital photographic apparatus, a digital camera, a database management system, a computer, a scanner, or a medical imaging apparatus, having means of implementing the insertion method, or the detection method, or including the insertion device, or the detection device, as disclosed above.
The advantages of the insertion device, of the detection device and method and of this digital apparatus are identical to those of the insertion method previously disclosed.
The invention also relates to recording means, integrated or not into the device, possibly removable, which store a program implementing the insertion or respectively detection method.